In current mobile terminals, there is a demand not only for smaller sizes and lighter weight, but also for functions that allow a user access to mobile communication services of different frequency bands through a single terminal. That is, there is a demand for a terminal with which a user may simultaneously utilize signals of multiple bands as necessary, from among mobile communication services of various frequency bands, such as the CDMA service based on the 824-894 MHz band and the PCS service based on the 1750-1870 MHz band commercialized in Korea, the CDMA service based on the 832-925 MHz band commercialized in Japan, the PCS service based on the 1850-1990 MHz commercialized in the United States, the GSM service based on the 880-960 MHz band commercialized in Europe and China, and the DCS service based on the 1710-1880 MHz band commercialized in parts of Europe.
Furthermore, there is a demand for a composite terminal that allows the use of services such as Bluetooth, ZigBee, wireless LAN, GPS, etc. In this type of terminal for using services of multiple bands, a multi-band antenna is needed, which can operate in two or more desired bands. The antennas generally used in mobile terminals include the helical antenna and the planar inverted-F antenna (PIFA).
Here, the helical antenna is an external antenna that is secured to an upper end of a terminal, and is used together with a monopole antenna. In an arrangement in which a helical antenna and a monopole antenna are used together, extending the antenna from the main body of the terminal allows the antenna to operate as a monopole antenna, while retracting the antenna allows the antenna to operate as a λ/4 helical antenna. While this type of antenna has the advantage of high gain, its non-directivity results in undesirable SAR characteristics, which form the criteria for levels of electromagnetic radiation hazardous to the human body. Also, since the helical antenna is formed protruding outwards of the terminal, it is difficult to design the exterior of the terminal to be aesthetically pleasing and suitable for carrying, but a built-in structure for the helical antenna has not yet been researched.
The inverted-F antenna is an antenna designed to have a low profile structure in order to overcome such drawbacks. The inverted-F antenna has directivity, and when current induction to the radiating part generates beams, a beam flux directed toward the ground surface may be re-induced to attenuate another beam flux directed toward the human body, thereby improving SAR characteristics as well as enhancing beam intensity induced to the radiating part. Also, the inverted-F antenna operates as a rectangular micro-strip antenna, in which the length of a rectangular plate-shaped radiating part is reduced in half, whereby a low profile structure may be realized.
Because the inverted-F antenna has directive radiation characteristics, so that the intensity of beams directed toward the human body may be attenuated and the intensity of beams directed away from the human body may be intensified, a higher absorption rate of electromagnetic radiation can be obtained, compared to the helical antenna. However, the inverted-F antenna may have a narrow frequency bandwidth when it is designed to operate in multiple bands.
The narrow frequency bandwidth obtained when designing the inverted-F antenna to operate in multiple bands is resultant of point matching, in which matching with a radiator occurs at a particular point.
Thus, there is a demand for an antenna that maintains a low profile structure and overcomes the drawback of the inverted-F antenna of narrow band characteristics for more stable operation in multiple bands.